Semi-conductor devices and methods of making same



Jan."14, 1958 s. M. CHRISTIAN ,8 0, 8

SWI-QQNDUCTQR DEVICES AND METHODS OF MAKING SAME Filed Dec. 1. 1953 INVENTOR.

Jaw 4:: M flyasr/n/v ATTORNEY SEMI-CONDUCTOR DEVICES AND METHODS OF MAKING SAME Schuyler M. Christian, Princeton, N. L, assignor to Radio Corporation of America, a corporation of Delaware Application December 1, 1953, Serial No. 395,381 10 Claims. (Cl. 317-239):

This invention relates to novel semi-conductor devices and methods for making them, and more particularly to such devices comprising a semi-conductor body of novel composition. It is known to make semi-conductor devices such as transistors comprising a body of semi-conductive germanium or silicon. For the production of satisfactory transistors the resistivity of the semi-conductive body is relatively critical. For most transistor applications employing germanium a resistivity within the range of about l'.5 ohm-cm. is desired. In certain instances, however, such as high frequency transistors the resistivity of the body should be as low as about 0.1 ohm-cm. It has been determined that intrinsically pure germanium and silicon have resistivities of approximately 60 ohm-cm. and 60,000 ohm-cm, respectively, at room temperature, and are not generally suitable for use in transistors.

Generally, conduction in a body is effected by charge carriers, electrons and holes, which. are available in intrinsically pure semi-conductive material in insufficient numbers to provide satisfactory transistor operation. Further, transistor operation is believed to be largely dependent upon an excess of charge carriers of one type over charge carriers of opposite type, i. e., a difference between the number of available electrons and the number of holes in the semi-conductor body must exist. lntrinsical'ly pure semi-conductive materials are believed to contain available electrons and holes in approximately equal numbers and, therefore, additional charge carriers of a selected type are ordinarily provided by doping the material. Doping is generally accomplished by adding a selected impurity material to a relatively pure semi-conductive material.

It has not been found practical to add impurity materials uniformly throughout a body in the solid state. Such addition would depend principally upon. diffusion, which at any temperature below the melting point of the body is an impracticably slow process- Generally, impurities are added to a molten mass of the material and the molten mass is thenfrozen.

A principal problem in thus producinga semi-conductive material. having adesiredresistivity concerns the uniformity of a body produced. The resistivity of a semiconductor body is inversely proportional to the impurity concentration. This uniformity, therefore, depends primarily upon the distribution of impurities throughout the body. Due to the fact that most impurity substances are more soluble in molten than in solid germanium or silicon, first frozen portions of a doped semi-conductor body generally contain less impurity material than do later frozen portions- In. view of these difficulties a process. called zone-levelling or zone-melting is commonly utilized.

The general theory of zone-levelling is discussed in an article by W. G- Pfann entitled, Principles of zone-melting, published in the Journal of Metals, July 1952. The process, briefly, comprises causing a relatively narrow molten zone to traverse the length of an elongated body of a relatively pure semi-conductive material. An impurity 2,820,185 Patented Jan. 14, 1958 2 material is placed initially in the molten zone and the segregation phenomenon is relied upon to distribute the impurity material relatively uniformly throughout the body as the molten zone traverses its length.

In the case of germanium; for example, the quantities of impurity materials to be added to such a molten zone in order to produce an ingot of germanium having a desired resistivity have in certaininstanc'es been determined. For example, it has previously been found that an ingot comprising 3 ohm-cm; germanium may be produced by zone l'evelling when: an impurity concentration of about 2X1'0' of antimony is established in the molten zone; Likewise, a concentration of about 10 ("1. part in 100,000) of indium may be employed toyield a germaniurn ingot having a resistivity of about 3' ohm-cm; Such ingots are preferably produced in the form of a single crystal.

These proportions are relatively small, and even when working with a relatively large body of germanium such as one wherein a: molten zone of'aboutlSO grams may be conveniently utilized, iris not readily practicable to meas ure the required quantity of doping material directly. It is instead usually preferred to dilute the. doping material by dissolving it in additional germanium. For examplair" the quantity of impurity material desired isa'bout' 02mg: it may be provided by adding about 20 mg; of an alloy consisting of 99% germanium and 1% impurity. new ever, even this quantity is relatively difficult to measure with a. high degree of. accuracy, and further, it is relatively difficul't' to provide an alloy of indium and germanium having the desired proportions with a high degree of uniformity.

Impurity materials. are classified generally into. three categories. Impurities such as antimony, which. when al.-- loyed with germanium provide free electrons in the conduction energy band, are calledn-t'ype or donor impurities. Materials such as indium, which when! alloyed with germanium provide acceptor centers or holes in. the valence energy band, are called p-type or acceptor i'mpuri? ties. Impurities such as lead, tin or. silicon. which do not affect theconductivity type of' germanium. are called neutral impurities.

It is an object of. the present invention to. provide. novel n-type semi-conductor materials suitable for. use inmaliing transistors.

Another object is to provide n-type semi-conductive germanium and silicon containing novel impurity ma.- terials.

Another object is to provide. n-type semi-conductive germanium and silicon. of novel composition.

Another object. is to provide a novel method. of making n-type semi-conductive germanium and silicon.

Another object is to provide semirconductor. devices ineluding improved semi-conductive materials.

Still another object' is to provide transistor devices comprising n -type semi-conductor bodies-of novel composition.

It has now been discovered that transistors and similar devices, having desirable: operating characteristics, can be made from single crystalline germanium or silicon con taini'ng. certain. small. percentages of bismuth. It has fur ther been discovered that the segregation coefficients of bismuth in germanium and in silicon are relatively small, of theorder of" 10 Therefore bismuth is particularly advantageous when used to dope germanium and silicon since relatively large and easily weighed quantities may be employed.

Previously, germanium doped with bismuth has been suggested for use inmaking point contact rectifier devices. However, bismuth has been incorporated in germanium in quantities of about .05% to 0.5% by Weight.

long, although this size is not critical.

The invention will be more fully described with reference to the drawing of which:

Figure 1 is a series of curves showing the variations of the relative impurity concentrations ofa series of single crystal ingots of germanium produced by the zonelevelling process.

Figures 2 and 3 are schematic, cross-sectional, elevational views of devices produced according to the instant invention.

Figure 1 illustrates one of the advantages of the instant invention as compared with previous practice. Curve A is an idealized curve that shows the variation of relative impurity concentration along the length of an n-type ger manium crystal doped with antimony and grown by the zone-levelling process. Curve B shows the relative impurity concentration of another n-type crystal grown in exactly similar manner but doped with bismuth according to the instant invention. The crystal doped with his muth is relatively more uniform over its length than the crystal doped with antimony. The resistivity of a semiconductor varies inversely according to its impurity concentration. Thus the bismuth-doped crystal is more uni.- form with respect to resistivity and a larger proportion of its length is useful to produce transistors.

This improved uniformity of impurity distribution results from the fact that the segregation coefficient f bismuth is relatively much smaller than the coeflicient of previously used impurity materials. The segregation coefiicient of an impurity substance in germanium, for example, may be defined as the ratio of the concentration of the impurity substance on the solid sides of the interface of a growing ingot to the concentration on the liquid side of the interface. (Concentration in solid+concentration in liquid.)

It has now been discovered that the segregation coefiicients of bismuth in germanium and in silicon are about 10- and 7Xl0" respectively. These coefficients may be compared to the coefficients for two other commonly used n-type impurities, i. e., antimony which has a mathcient in germanium of about 5 l0- and arsenic which has a coefiicient of about 6 lO The relatively small segregation coefiicient of bismuth not only provides a greater uniformity in a crystal doped with bismuth than in previous crystals, but also permits the addition of relatively large and easily weighed quantities of bismuth to the material in process. The relatively high atomic weight of bismuth is also advantageous in this respect.

A single crystal of germanium having a resistivity of about 3 ohm-cm. may be produced according to the instant invention by the zone-levelling process generally in accordance with the method described in the article by W. G. Pfann mentioned heretofore, as follows:

A solid ingot of semi-conductive germanium of relatively high purity and having a resistivity of about 40-50 ohm-cm. is placed in an elongated crucible which may be of silica. Somewhat less pure germanium may be utilized but more uniform results are obtained if the resistivity is at least about 35 ohm-cm. The germanium ingot may conveniently be about 2 cm. x 2 cm. x 50 cm. The crucible may conveniently be of about the same size and shape as the germanium ingot although this also is not critical. About 30 mg. of bismuth is placed at one end of the germanium. It is preferred to place the bismuth within a small depression or cavity upon a surface of the germanium not adjacent the silica crucible in order to permit the bismuth to dififuse evenly throughout the molten zone and to minimize any absorption or reaction of bismuth into or with the crucible.

A relatively small single crystal of germanium is placed in the crucible adjacent the bismuth-bearing end of the ingot. This crystal acts as a seed to guide the growth of iheentire ingot into a single crystal. If it is desired to product'a polycrystalline bismuth-doped ingot the seed crystal may be omitted.

The crucible bearing the germanium charge is placed within a silica tube within a zone-melting furnace. The silica tube is provided with means to maintain a non-oxidizing atmosphere such as dry hydrogen about the germanium during the process.

A zone about 4.5 cm. long is melted at the bismuthbearing end of the charge and maintained in a molten state at about 960 to 1000 C. for about 5-15 minutes to permit the bismuth to diffuse evenly throughout the zone and to insure complete melting of the germanium. This zone includes a portion of the seed crystal in order to insure growth of a single crystal structure as the germanium freezes. The molten zone is then caused to progress at a uniform rate of about 1 mm. per minute from the seed end of the ingot to the far end. As the zone progresses, the solid polycrystalline ingot is slowly melted and a single crystal of germanium grows attached to the seed crystal. A relatively small proportion of the bismuth initially placed in the molten zone is uniformly distributed along the length of the grown crystal except for the last frozen 4.5 cm. portion which comprises a relatively large proportion of the initially added bismuth.

The variation of relative impurity concentration in the major portion of a crystal grown in this manner is shown in Curve B of Figure l and may be compared with that of a crystal grown in similar manner but doped with antimony as shown in Curve A.

In the production of certain transistor devices, especially those designed for high frequency applications, it is often desired to provide a semi-conductive germanium body having a resistivity of about 0.1 ohm-cm. Such a. material may be readily provided according to the practice of the instant invention by a method exactly similar to that described heretofore, except that about 900 mg. of bismuth is added to the molten zone. A relatively large portion of the crystal grown by this method is sufficiently uniform to be useful in the production of transistor devices.

In a similar manner semi-conductive germanium may be produced having a resistivity of about l0 ohm-cm. such as may be desired for use in certain devices. The amount of bismuth to be added to the molten zone in this case is about 9 mg., and the remainder of the process may be exactly as heretofore described.

The last grown portions of crystals or ingots produced in accordance with the instant invention, although not generally suitable for transistor production, are not wasted but are utilized to make new ingots or crystals. Since these last portions include almost all of the bismuth initially added to the ingots they may be conveniently used as the starting portions of new ingots or crystals, thus saving additional impurity measuring steps.

The practice of the instant invention is advantageous in the production of semi-conductive silicon. Silicon has a melting point of about 1420 C., and many previously used n-type impurities boil below this temperature. It has been found difficult, therefore, to dope silicon according to previous practice. Bismuth. on the other hand, does not boil at the melting point of silicon, and so is relatively easy to maintain in solution in molten silicon.

It should be noted that the advantages of the instant invention are not limited to the zone-levelling process. In the cases of both silicon and germanium the practice of the invention is advantageous in other processes such as the vertical crystal pulling technique.

In doping a molten zone to grow a crystal of bismuthdoned germanium, the concentration of bismuth established in the molten zone is critical. For example, to produce a crystal having a resistivity of about 3 ohm-cm, it is known that the impurity concentration in the frozen crystal should be about 10- i. e., the crystal should include one efiective impurity center for each 10 atoms of the pure crystal material. Each atom of a doping material provides one impurity center. The segregation coefiicient of bismuth is about 10-, and therefore if the molten zone comprises a bismuth concentration of about a crystal grown from the zone will have the desired resistivity. For example, in the process heretofore described a molten zone 4.5 cm. long and having a cross section 2 cm. square was described. This molten zone comprises about 100 grams of germanium. The atomic weights of germanium and bismuth are about 73 and 209 respectively. Therefore a quantity of about mg. of bismuth is utilized to provide a bismuth concentration of about 10 According to such simple, known relationships the quantity of bismuth required in a zone-levelling process to produce a semi-conductor body having a desired resistivity may be roughly determined. The precise quantity is preferably determined empirically since may variables must be taken into account. For example, the apparent, or effective value of the segregation coefficient may be varied over relatively wide limits by varying such factors as the speed of crystal growth, the temperature of the melt, or the impurities present in the semi-conductive material immediately prior to the process.

In general, to provide semi-conductive germanium or silicon of the most generally useful resistivity, within the range of 0.1 to 10 ohm-cm, an atomic impurity concentration of about 10-' to 10* must be established in the material. In the practice of the instant invention a desired impurity concentration in the solid material may be provided by maintaining an atomic concentration of bismuth in the melt of about 10- to l0- It should be understood that the concentrations described in the preceding paragraph are orders of magnitude only, and are not exact measures. Exact figures are not given because of the relatively large number of variables that may atfect the process.

It has been found that the purity of the bismuth utilized in the practice of the invention is not critical. Satisfactory results are obtained with the ordinary C. P. grade of bismuth such as, for example, bismuth of 99% purity.

According to a preferred embodiment of the invention a transistor may be produced utilizing a wafer of germanium doped with bismuth. The wafer may be cut by known techniques from a single crystal grown by the zone-levelling process.

A typical device utilizing bismuth-doped germanium is illustrated in Figure 2. Figure 2 shows an alloy junction transistor comprising a base water 4 of bismuth-doped germanium produced according to the instant invention and having a resistivity of about 3 ohm-cm. The wafer may conveniently be about 0.25 x 0.25 x .006 thick. A pair of electrodes '7 and 8 of indium are fused to opposite surfaces 5 and 6 respectively of the wafer. Within the wafer there are disposed two opposite p-n rectifying junctions 9 and 11, each adjacent one of the electrodes. Two electrical leads 12 and 14 make contact with the respective electrodes. A metal tab 16 is attached to the wafer by means of a non-rectifying solder connection 17. When utilized in a circuit the smaller electrode 7 may conveniently be employed as an emitter, the larger electrode 8 as a collector and the wafer 4 as a base.

The manner of operation of a device such as that described in the preceding paragraph is not critical according to the present invention. In all ways that can presently be determined the performance of a device produced in accordance with the instant invention is comparable to the performance of previous such devices that utilize n-type semi-conductive germanium doped with materials other than bismuth.

A device utilizing a body of relatively low resistivity germanium produced according to the invention is illustrated in Figure 3. Figure 3 shows a point contact transistor suitable for high frequency operation. This transistor comprises a base wafer 22 of bismuth-doped germanium about .04" x .04" x .02" and having a resistivity of about 0.1 ohm-cm. An electrical lead 26 is bonded to one surface 23 of the wafer by means of a non-rectifying solder connection 28. Upon the opposite surface 25 of the wafer two closely spaced relatively hard, pointed metallic wires 30 and 32 are pressed. The ends of the wires are sharpened to chisel points so that the areas of contact between the wafer and the wires are minimized. The ends of the wires contact the wafer at two points about .0005 apart. One of the wires may be advantageously employed in a circuit as an emitter electrode, the other wire as a collector electrode, and the lead 26 may conveniently serve as a base connection.

There have thus been described semi-conductive germanium of novel composition, methods of making it and devices utilizing such germanium.

What is claimed is:

1. In a semi-conductor device comprising a semi-conductor body of a material selected from the class consisting of germanium and silicon and electrodes connected to said body, the improvement consisting of said body comprising a region of n-type semi-conductive material having an excess of n-type over p-type impurity concentration, said excess being of the order of 10- to 10*, and said excess impurity being atoms of bismuth.

2. The invention according to claim 1 in which said excess is of the order of 10- and said resistivity is about 2-5 ohm-cm.

3. The invention according to claim 1 in which said semi-conductive material is n-type germanium.

4. In a transistor comprising a plurality of spaced electrodes in contact with the surface of a semi-conductor body, the improvement consisting of said body having a region of n-type semi-conductive material, said material being selected from the class consisting of germanium and silicon having a resistivity of 0.1 to 10 ohm-cm. and having an excess of n-type over p-type impurity concentration, said excess being of the order of 10- to 10- and said excess impurity being atoms of bismuth.

5. In a transistor comprising a semi-conductor body of a material selected from the class consisting of germanium and silicon having adjacent regions of n-type and p-type conductivity, respectively, a p-n rectifying junction disposed intermediate said regions, and an electrode attached to said p-type conductivity region, the improvement consisting of said body having a region of n-type semi-conductive material, said material having a resistivity of 0.1 to 10 ohm-cm. and having an excess of n-type over p-type impurity concentration, said excess being of the order of 10- to 10* and said excess impurity being atoms of bismuth.

6. The invention according to claim 5 in which said semi-conductor body consists essentially of germanium.

7. A semi-conductor body of a material selected from the class consisting of germanium and silicon said body being of substantially single crystal structure consisting essentially of an excess of n-type over p-type impurity concentration, said excess being about 10-' to 10- and said excess impurity being atoms of bismuth.

8. A body according to claim 7, said body consisting essentially of germanium.

9. A body according to claim 7, said body consisting essentially of silicon.

10. A body according to claim 7, said body having a resistivity of 0.1 to 10 ohm-cm.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN A SEMI-CONDUCTOR DEVICE COMPRISING A SEMI-CONDUCTOR BODY OF A MATERIAL SELECTED FROM THE CLASS CONSISTING OF GERMANIUM AND SILICON AND ELECTRODES CONNECTED TO SAID BODY, THE IMPROVEMENT OF SAID BODY COMPRISING A REGION OF N-TYPE SEMI-CONDUCTIVE MATERIAL HAVING AN EXCESS OF N-TYPE OVER P-TYPE IMPURITY CONCENTRA- 